.case_contents {
    margin-bottom: 50px;
}

.layout_04 img {
    float: left;
}
.layout_04 img:nth-last-child {
    clear: left;
}
.layout_04 img:first-child:nth-last-child(1) {
    width: 100%;
}
.layout_04 img:first-child:nth-last-child(2),
.layout_04 img:first-child:nth-last-child(2) ~ img{
    width: 50%;
}
.layout_04 img:first-child:nth-last-child(3),
.layout_04 img:first-child:nth-last-child(3) ~ img{
    width: 33.3%;
}
.layout_04 img:first-child:nth-last-child(4),
.layout_04 img:first-child:nth-last-child(4) ~ img{
    width: 25%;
}